The industrial preparation of cellulose ethers takes place nowadays exclusively under heterogeneous reaction conditions whereby cellulose is initially activated with concentrated alkali liquor and then reacted with alkyl halides or epoxy alkyl compounds at elevated temperature. Activation is required on the one hand to loosen the partially crystalline structure of the cellulose and to increase the accessibility of the hydroxyl groups, and on the other hand to polarise the hydroxyl groups of the cellulose to the extent that nucleophilic substitution of the halogen on the alkyl halide or ring cleavage in the case of epoxides followed by addition becomes possible. In order to improve the uniformity of substituent distribution and hence to improve the product properties, the reaction is often carried out in the presence of inert organic solvents such as, e.g. 2-propanol.
Disadvantages of carrying out the reaction heterogeneously throughout the process are that:    1) a high alkali concentration is required to swell and activate the cellulose even in reactions requiring only catalytic quantities,    2) a decrease in the molecular weight and a high salt load during neutralisation of the liquor is consequently unavoidable,    3) it is not possible to prepare completely soluble and yet high-viscosity cellulose ethers with low degrees of substitution,    4) as the morphological structure of the cellulose is largely maintained, it is not possible to achieve a satisfactory uniform etherification along and between the polymer chains, and unsubstituted monomer units always occur in addition to di- and tri-substituted anhydroglucose units, even in the case of soluble, relatively highly substituted products,    5) solubility properties, thermal stability or compatibility of the cellulose ethers with respect to ionic or nonionic substances of a low or high molecular weight nature are frequently unsatisfactory and restrict the fields of application,    6) a regioselective homogeneous derivatisation of free OH groups in certain positions of the anhydroglucose unit is not possible.
In order to overcome such disadvantages due to the heterogeneous synthesis, various aqueous and non-aqueous cellulose solvent systems have been used in recent decades for the etherification of cellulose in the homogeneous phase. Apart from achieving a more uniform substituent distribution, the aim was at the same time to provide access to soluble cellulose ethers with longer-chain and extensive substituents. Proposed solvent systems which are sufficiently stable under the required reaction conditions and are also inert towards the reagents to be used were mainly quaternary ammonium bases (U.S. Pat. No. 2,087,549), mixtures of sulfur dioxide/dimethylamine/dimethylsulfoxide (A. Isogai, A. Ishizu, J. Nakano: J.Appl. Polymer. Sci. 31(1986) p. 341-352), mixtures of dimethylsulfoxide/paraformaldehyde (U.S. Pat. No. 4,024,335), mixtures of N,N-dimethylacetamide/lithium chloride (U.S. Pat. No. 4,278,790) and N-methylmorpholine-N-oxide (NMMNO) (reviews in: Methoden der Organischen Chemie (Houben-Weyl), vol. E20-Makromolekulare Stoffe, eds. H. Bartel and J. Falbe, Georg Thieme Verl., Stuttgart-New York, 1987, p. 2086-2093 and B. Philipp, B. Lukanoff, H. Schleicher, W. Wagenknecht: Z. Chem. 26(1986)2, p. 50-58).
These cellulose solvent systems have been unable to become established hitherto on an industrial scale, in some cases because of the limited dissolving power, particularly with respect to high molecular weight starting celluloses and relatively high cellulose concentrations, the difficulty of recovering the components, or for cost reasons, despite the wide possibilities of derivatisation.
The most promising cellulose solvent for industrial use has recently proved to be NMMNO, though commercial interest has been shown almost exclusively in forming cellulose to fibres and films (U.S. Pat. No. 3,447,956, U.S. Pat. No. 4,196,282, EP 452610, WO 95/11261).
Homogeneous etherification reactions of cellulose in melt solutions of cyclic amine oxides, particularly of NMMNO, in the presence of organic solvents, preferably DMSO, as diluent to cyanoethylcellulose or cellulose oxyethyl methyl ketone using the vinyl compounds acrylonitrile and methyl vinyl ketone were described for the first time by Johnson (U.S. Pat. No. 3,447,939), operations being carried out in a disadvantageous manner with very low cellulose concentrations, extremely high reagent use quantities and high reaction temperatures which promote the decomposition of the N-oxide. In the case of cyanoethylation, the use of benzyltrimethylammonium hydroxide is mentioned as etherification catalyst. In all the reactions mentioned here, strongly coloured cellulose derivative solutions were initially obtained. The reagent yields were extremely low and the resulting cellulose derivatives were degraded to a greater or lesser extent, for which reason this process principle and the products obtained herewith appear to be unsuitable for industrial use.
It was later found that by adding various stabilisers such as, e.g., propyl gallate (EP-B 0 047 929), phenylpropionates, thioethers or disubstituted phenylene diamine (DE-OS 42 446 099), phosphates or phosphonates (WO 83/044415), basic substances (DD 158656), DD 218104), particularly amines (U.S. Pat. No. 4,290,815), degradation of cellulose during dissolution in NMMNO and the decomposition of NMMNO itself can be largely prevented.
The preparation of high-swelling carboxymethyl celluloses with a low degree of substitution by reacting cellulose with monochloroacetic acid or Na monochloroacetate in the NMMNO system using sodium hydroxide solution as base is claimed in DD-PS 207 380. It is not mentioned here that the system has a strong tendency to coagulate when sodium hydroxide solution is added, and that non-uniformly substituted products are obtained. This coagulating effect of sodium hydroxide solution is also observed with most of the other above-mentioned non-aqueous cellulose solvents, which is a decided disadvantage of these well known so-called homogeneous etherification processes. The use of powdered caustic soda (U.S. Pat. No. 4,278,790), Na alcoholate and NaH (A. Isogai, A. Ishizu, J. Nakano: J. Appl. Polymer Sci. 29(1984) p. 2097-2109) as catalyst in the lithium chloride/dimethyl acetamide system did not bring the desired success either and led to heterogeneities during the reaction to partially substituted derivatives. The catalytic effect of NMMNO during the hydroxyalkylation of mannan as a model substance was analysed by Seneker and Glass (Polymeric materials science and engineering 52 (1985) p. 39-43) and a preferential substitution on the C-6 OH group was assumed for cellulose.